The theists have a relatively easy time of defining evil: evilness is rooted in committing sins. Since a sin is a crime against god, the non-theist has a harder time of it. If you do not believe in a moral setting judgmental god, then disobeying his edicts is not possible.
So agnostics do not believe that evil exists? No – we are certain that it does, it is just that we take great care in defining just what acts qualify as being evil.
As does the theist, if you think about it. Burning witches for example is now considered by all to be very un-kosher. Downright evil if you are non-theist, an unfortunate lapse by apologists of canon history. Slavery? Beyond the pale – except not long ago, and still in pockets around the globe -many sane and sensible people considered it to be a very acceptable practice.
Hitler! He was evil, wasn’t he? Yes. But not to him. He seemed to honestly believe that he was doing good for humanity as a whole. Genetic cleansing was a painful, but necessary duty in order to give us a better humanity. He was clearly totally wrong: but you would never be able to convince him of the error of his ways.
Which means that evil has two dimensions: it changes over time, and it changes over societies.
Evil acts, taking out theistic notions, all seem to involve dealing out death, pain, misery and anguish without any just cause for doing so. It is the just cause that is the moving target. Today’s judicial execution of criminals given due process may be seen in some future time as an act of evil revenge committed by a mindless society. Or one that invokes a sense of compassion and grief not known to today’s way of reacting to history’s catalog of evil done in goodness’s name.
And it seems to be almost a self evident truth that when numbers amass to commit evil acts, the effect is exponential. When a lone gunman slays innocent schoolchildren in unspeakable outrage, the news of it gives a grief that is so deep and profound it numbs the mind. When a whole section of a foreign society makes its plans to commit similar senseless slaughter on others, the natural rage it invokes leads to the mobilization of militia, arming of drones, and taking preemptive strikes to prevent such acts being successful.
It would seem to me that the ability to commit evil acts is totally dependent upon a person’s belief system. If you do not believe the act is evil, you will have no difficulty in carrying it out. And should you live in a society where all agree that something unspeakable is necessary, the members reinforce each others ability to commit harm against all outsiders.
It is germane to ask whether one can one do an evil act to oneself? The theist may make some sort of case that you can. If god says you are not to do something to yourself, and you do it, then you are sinning. For the theist, the division between sin and evil is so narrow as to make it possible that you can do evil things to yourself.
The non-theist would argue that rational beings only do rational things to themselves. To do something irrational to yourself is a symptom of breakdown in your rationality, and indicates a course of action, or cure, that is based on mental health rather than invoking the forgiveness of a moral setting, judgmental god.
However, theist and non-theist can agree that acts on third parties that cause death or pain that are carried out for personal gratification, or other sources of unjustified and unjustifiable reasons, will almost invariably be actions that amount to acts of evil.
To bring all these points into personal focus, I think it is right to say that we are allowed to carry any belief system we wish – indeed, our belief system at any one moment in time is the sum of our life experiences to date, and any other belief system is unthinkable. But society can insist that each personal belief system shall not be one that allows the harming other human beings. And justifiably so, in any society whose members wish to live in a civilized manner.
Evil comes from believing that others do not have the right to life. And if their belief system is not the same same as ours, the belief that forced change is permitted.
The agnostic way is to acknowledge that we do not have to like every single other human on the planet – but dislike of one or more does not give us license to do them harm. And harming others for personal gratification is, when mild, totally immoral – and when not mild, an act that qualifies for any definition you wish to make for evil.